As a conventional active noise control device, a device is disclosed which is capable of reducing a noise (a target sound) from a noise source in a wide range by producing a synthesized control sound with an opposite phase to the noise, which is produced by multiple speakers arranged around the noise source (see, Patent literature (PTL) 1 for example).
FIG. 1A illustrates a cross-sectional view showing a vertical section of an air-conditioning indoor equipment 1 including the conventional active noise control device as described in PTL 1, and FIG. 1B illustrates a plan view (a bottom view) showing a air-conditioning indoor equipment 1 depicted from the bottom of it in FIG. 1A.
As shown in FIG. 1A and FIG. 1B, the air-conditioning indoor equipment 1 includes a turbofan 2 which generates a noise, heat exchangers 3, a suction grille 4 provided in the bottom of the air-conditioning indoor equipment 1, and a sound generation unit 5 for emitting the synthesized control sound with the opposite phase in the same direction as a direction of propagation of the noise generated by the turbofan 2 (in a downward direction in FIG. 1A).
As shown in FIG. 1B, the sound generation 5 includes five speakers 5a to 5e which are provided around an air flow channel 6 and have an array arrangement. A distance d between the speakers 5a and 5e is less than a one-half wavelength of sound at the highest frequency of the noise generated by the turbofan 2. Like the distance d between the speakers 5a and 5e, a distance h between the speaker 5a and the turbofan 2 is also less than a one-half wavelength of sound at the highest frequency of the noise. Thus, since the turbofan 2 and the speaker 5a are placed close to each other and the speakers 5a to 5e are also placed closely so that both of the distances h and d are less than the wavelength of sound at the highest frequency of the noise, a propagating wave front of the noise roughly coincides with a propagating wave front of the control sound with the opposite phase which is synthesized by the speakers 5a to 5e. Therefore, the noise can be widely reduced in a three-dimensional space.